1. Field of the Invention
The present invention relates to an improvement of a motor-driven mold clamping device for a motor-driven injection molding machine as well as to an improved mold clamping method.
2. Description of the Related Art
Motor-driven injection molding machines have gradually replaced hydraulic machines, and have increasingly been used in recent years. One reason lies in their simple configuration as compared with the hydraulic injection molding machines because of the lack of need of a hydraulic pump, hydraulic tubes and valves. In addition, servo motors used for the source of power allows the easier control of the motor-driven injection molding machines. The servo motors are used in most cases for an injection device and a mold clamping device.
As far as mold clamping devices are concerned, it is often based on a toggle system. The toggle system uses a toggle mechanism to double the force generated by the servo motor which is then transmitted to a mold by means of a toggle link. These types of mold clamping devices are undergoing changes and refinements. An example of an improved toggle-operated mold clamping device is disclosed in Japanese Patent Publication No. 1-22135. The disclosed mold clamping device comprises a servo motor and a position detector for detecting a rotation position of the servo motor. The mold clamping device further comprises a conversion mechanism for converting the rotation movement of the servo motor into a linear movement. The conversion mechanism has a ball screw mechanism. The conversion mechanism is used for driving the toggle mechanism and the position detector detects a position of a movable mold, to carry out control operation of the mold clamping. Upon the mold clamping, the servo motor is driven with a microcurrent flowing therethrough.
For the toggle mechanism, the reason the microcurrent is used is to generate a sufficient clamping force with the phenomenon known as knicking reduced as much as possible. This provides a large toggle magnification factor and thus allows a smaller output of the servo motor. The mold clamping devices using the toggle mechanism of the type described advantageously require only a small electric power consumption. As will be described more in detail below, the smaller the knicking is, the shorter the distance from the dead point of the toggle mechanism.
However, the smaller knicking results in a larger effect of a frictional force on junctions and contacted portions of mechanical parts forming the mold clamping device. This therefore increases an operational hysteresis. Such a large operational hysteresis has an adverse effect on the accuracy of control for the mold clamping force provided by the servo motor.
Therefore, an object of the present invention is to improve the accuracy of control for the mold clamping force while reducing the effect of a frictional force on junctions and contacted portions of mechanical parts forming the mold clamping device.
A motor-driven mold clamping device according to the present invention comprises a toggle mechanism for use in advancing and retracting a movable platen; and a servo motor adapted to drive the toggle mechanism via a ball screw mechanism.
According to an aspect of the present invention, the motor-driven mold clamping device controls a mold clamping force with a knicking in the toggle mechanism being in a predetermined range and the servo motor driven with a current which is not smaller than 20% of a rated current therefor.
A method for clamping a mold in a motor-driven manner according to the present invention clamps the mold by means of a toggle mechanism operated by a servo motor. In this method a mold clamping force is controlled with a knicking in the toggle mechanism being in a predetermined range and the servo motor driven with a current which is not smaller than 20% of a rated current therefor.